Allergic contact dermatitis (ACD) is one of the most common and costly health problems in industrialized countries. Currently, more than 100,000 chemicals are in commercial use and another 2,000 new chemicals and numerous new-product formulations are tested or marketed every year. All of these materials must be screened for allergenicity. During Phase I, a human dendritic cell based assay was developed which gives sensitive and specific responses to allergens. The non-optimized assay is approximately a factor of 10 less expensive than current animal based tests. In addition, the assay avoids animal welfare issues and uncertainty related to species extrapolation. During Phase II, thedatabase of materials tested will be expanded and the in vitro assay method and prediction model will be finalized. The reproducibility of the assay will be assessed and, after demonstration of interlaboratory transferability, a multi-lab GLP validation study will be performed. Finally, an optimized assay kit which further enhances the economic advantages of the in vitro method will be developed. Allergic contact dermatitis, in vitro testing, plasmacytoid DC. A predictive test system for assessing the allergic contact dermatitis potential of chemicals will have far reaching application in industries involved in cosmetic, personal hygiene and topical pharmaceutical products. Evaluation of allergenicity is important to minimize the hazards of occupational and public exposure to chemicals and products. The proposed human cell based assay will provide a sensitive and specific method of allergen screening without the use of laboratory animals and at a reduced cost.